The invention relates to liquid filters, and more particularly to a service interval change indicator more accurately reflecting filter life.
Liquid filters, including fuel filters, typically have a vertically axially extending housing having an annular filter element extending axially between top and bottom ends and having an inner hollow interior and an outer annular space between the filter element and the housing. The housing has an inlet to the annular space, and an outlet from the hollow interior. Liquid is filtered by flowing from the annular space through the filter element into the hollow interior. The annular space is viewable through the housing, e.g. through a transparent housing side wall, such that an operator or service technician can see the level of liquid in the annular space as an indication of when to replace the filter element. The higher the level of liquid in the annular space the greater the pressure drop across the filter element and hence the greater the plugging of the filter element.
In many applications, the liquid or fuel level, including the rise thereof in the noted annular space, does not accurately reflect filter life. For example, in one application, fuel level in a clear housing reaches the top with 2xe2x80x3 Mercury, Hg, restriction, while the filter element is capable of 8xe2x80x3 Mercury, Hg, restriction. Hence, using fuel level in the noted annular space of the clear housing as an indicator to change the filter element results in a premature such change. This is objectionable because of the less than full life usage of the filter element, the more frequent filter element changes, and the corresponding higher overall cost thereof.
The present invention addresses and solves the above noted objections. In one aspect, the invention delays the rise in liquid level in the noted annular space to correct the otherwise premature indication of a need to change the filter element. In another aspect, liquid level in the noted annular space is allowed to rise to controlled levels providing advance and more accurate indication of a forthcoming need to change the filter element.